1. Field of the Invention
The present invention relates to a method for manufacturing an SOI (Silicon On Insulator) substrate based on a SIMOX (Separation by IMplanted OXygen) method, and more particularly to a method for manufacturing an SOI substrate having a gettering capability.
2. Description of the Related Art
As one of substrates for semiconductor devices, there is an SOI substrate having a silicon layer (which will be referred to as an SOI layer hereinafter) formed on a buried silicon oxide film (which will be referred to as a BOX layer) as an insulator film. Since the SOI layer in a wafer front surface layer portion serving as a device fabrication region is electrically separated from the bulk substrate through the buried silicon oxide film layer (BOX layer), this SOI substrate has characteristics that a small parasitic capacitance, a high radiation-proof effect, and others. Therefore, a high-speed low-power-consumption operation and an effect of, e.g., avoiding a software error are expected, and the SOI substrate appears promising as a substrate for a high-performance semiconductor device.
As a typical method for manufacturing this SOI substrate, there is a wafer bonding method or a SIMOX method. The wafer bonding method is a method of forming a thermal oxide film on a surface of one of, e.g., two single-crystal silicon substrates (silicon wafers), then contacting the two wafers against each other through the formed thermal oxide film, carrying out a bonding heat treatment to increase a bonding strength, and thereafter carrying out, e.g., mirror polishing with respect to one wafer to reduce a film thickness, thereby manufacturing an SOI substrate.
On the other hand, the SIMOX method is a method of ion-implanting oxygen into a single-crystal silicon substrate, then performing a high-temperature heat treatment (oxide film-forming heat treatment) to react the implanted oxygen with silicon, and forming a BOX layer to manufacture an SOI substrate.
Specifically, for example, an oxygen ion is implanted into a single-crystal silicon substrate heated to approximately 300 to 500° C. from one surface. The method is generally classified into a high-dose method of setting an acceleration voltage to 150 to 200 keV and implanting the oxygen ion having a dose amount of approximately 1 to 2×1018/cm2 or above and a low-dose method of implanting the oxygen ion having a dose amount equal to or below this value as ion implantation conditions. Further, there is also a method called MLD (Modified Low Dose) by which an oxygen ion of approximately 2 to 4×1017/cm2 is implanted and then damage implantation of approximately 1 to 5×1015/cm2 is carried out to facilitate an ITOX (Internal Thermal Oxidation) effect. After the oxygen ion is implanted, a high-temperature oxide film-forming heat treatment (generally, 1300° C. or above) is performed in, e.g., an inert gas containing 50% or below of oxygen so that the implanted oxygen (oxygen-ion-implanted layer) is changed into an oxide film (BOX layer) having a thickness of approximately 100 to 200 nm.
As compared with the wafer bonding method, manufacture of the SOI substrate based on such a SIMOX method is considered to have advantages that a manufacturing process is simple, the SOI substrate can be manufactured from one single-crystal silicon substrate without requiring two wafers, and hence a cost of the substrate can be reduced.
However, in this SIMOX substrate, a damage is produced between the BOX layer and the silicon layer immediately below the BOX layer at the time of BOX formation based on ion implantation, and a metal impurity in the silicon layer immediately below the BOX layer serving as a trapping source of a metal, especially Ni mainly introduced from an SiC component in a heat treatment furnace during the high-temperature oxide film-forming heat treatment tends to have a high concentration. In such a case, there is a problem that electrical characteristics of the BOX layer, especially a dielectric breakdown voltage is degraded to deteriorate device characteristics.
Thus, to suppress an increase in a Ni concentration in the silicon layer under the BOX layer, forming a gettering layer having a capability (gettering capability) of trapping a metal impurity including Ni and others to be removed from a region serving as an active layer of a semiconductor device is important.
Therefore, there is a method of using a substrate having a polysilicon layer formed as a gettering layer on a back surface thereof. However, this method has problems of an increase in a price of the substrate leading to a decrease in a cost merit and poor flatness of an SOI substrate because of bad uniformity of a film thickness of the polysilicon layer.
Further, when a substrate having a crystal defect introduced to a back surface thereof due to, e.g., ion implantation is used as a starting material, the introduced crystal defect recovers during the high-temperature oxide film-forming heat treatment to avoid contribution as a gettering layer in some cases.
To prevent the defect from recovering during the heat treatment, the number of the crystal defects to be introduced must be increased, namely, a dose amount must be increased in case of ion implantation. Furthermore, when the high-temperature oxide film-forming heat treatment is performed in an oxidizing atmosphere, since thick oxide films are formed on both front and back surfaces, each crystal defect introduced in a shallow portion on the back surface is turned to the oxide film to avoid contribution as the gettering layer in some cases. Thus, the crystal defects must be deeply introduced beyond an oxide film-forming thickness in order to maintain a defect layer even after forming the oxide film. That is, in case of ion implantation, an acceleration voltage must be increased. Performing such ion implantation with a large dose amount and a high acceleration voltage does not lead to a manufacturing method having a simple process and a low cost.
Besides, as the gettering method of the SIMOX method, there is presented a method of performing a high-temperature heat treatment in a partial SIMOX substrate and then carrying out laser irradiation, ion implantation, sand blast, or polycrystal silicon layer deposition with respect to a back surface of the substrate to introduce crystal defects or crystal damages (see Japanese Patent Application Laid-open No. H05-82525). Moreover, there is also presented a method of amorphizing an SOI layer by implanting an argon ion to form a gettering region in the SOI layer based on the SIMOX method (see Japanese Patent Application Laid-open No. H10-214844).
However, it cannot be said that the above-explained methods do not have simple processes, and a manufacturing method with a low cost cannot be realized.